This invention relates to arc welding and more particularly to pulsed direct current arc welding. Specifically, this invention relates to a method of pulsed direct current arc welding wherein special pulses of positive direct current are used to weld together work pieces, especially work pieces made of aluminum, without using a flux.
There are many situations in which it is desirable to arc weld together two pieces of metal. For example, heat exchangers for air conditioning systems may be made from sections of thin wall aluminum tubing which are joined to provide a continuous circuit for the flow of a refrigerant. The sections must be joined so that there are no leaks. One method of accomplishing this is by arc welding.
Moisture, grease, oil films, and other foreign materials on the surface of the metal can cause arc welds of poor quality. Metakls which form difficult to reduce oxide films on their surfaces, such as aluminum, magnesium, and beryllium copper, pose an especially difficult problem with respect to making a weld of good quality. The quality of the weld is affected because the foreign materials and oxides can become entrapped in the weld and cause porosity that may affect weld strength and ductility. Generally, a longer lasting, stronger and less porous weld results if these surface contaminents are removed prior to welding. Degreasing for removal of foreign materials other than oxides may be done with commercial solvents by wiping, spraying, dipping, vapor degreasing, or steam cleaning the metal, followed by a hot water rinse. The difficult to reduce oxides typically only may be removed by using certain chemical solutions which are difficult to handle. Some metals, such as aluminum, magnesium, and beryllium copper instantaneously reform oxide films when exposed to air. Thus, these metals are particularly difficult to arc weld since even if the oxides on these metals are removed prior to welding the metals must be maintained in an atmosphere in which oxides do not reform or other measures must be taken to prevent oxides from reforming if good quality welds are to be consistently made.
One method of dissipating difficult to reduce oxides on metals such as aluminum, magnesium, and beryllium copper is by using a flux to break up the oxides on the surfaces of the metals during the arc welding process. Typically, a nonmetal chlorine or fluorine base flux is applied to the weld joint as the metal is welded. The flux is corrosive and is not always compatible with the environment in which it is used. After a weld is made flushing is necessary to clean the welded area. This is time consuming and costly and does not provide complete assurance that all contamination is removed. An example of a method of direct current arc welding in which a flux is used is disclosed in U.S. Pat. No. 3,552,412 to Bell, et al.. This patent relates to a method of welding aluminum wherein Solar 202, an aluminum welding flux, is used to break down aluminum oxides.
Fluxless methods of arc welding metals, such as aluminum, magnesium, and beryllium copper, which form difficult to reduce oxide films on their surfaces, are known. For example, alternating current arc welding techniques, similar to the methods described in U.S. Pat. No. 3,894,210 to Smith, et al. and U.S. Pat. No. 3,818,177 to Needham, et al. can be used to weld such metals without using a flux. However, these alternating current arc welding techniques are intended primarily for welding relatively thick pieces of metal which do not require precise control of the power flow to the metal during the arc welding process. These techniques are not ideal for welding certain materials, such as thin wall aluminum tubing used in making heat exchangers for air conditioning systems.
It is preferable when welding thin wall aluminum tubing and other such materials to use a direct current arc welding process whereby power flow to the work pieces may be precisely controlled. Also, direct current arc welding has other benefits such as prolonging the life of electrodes used in the welding process. While direct current arc welding has these and other advantages for welding certain materials, such as thin wall aluminum tubing, direct current arc welding has not heretofore provided the capability of dissipating oxides which is possible when using certain alternating current arc welding techniques. Therefore, it is desirable to provide a fluxless arc welding method having the advantage of precise power control and other advantages inherent in direct current arc welding and having the oxide dissipation capability of certain alternating current welding techniques.